In a semiconductor manufacturing apparatus (such as an exposing apparatus), which manufactures a memory with a high density of a CPU with high specifications, required exposure resolution is not more than 0.20 [μm]. Thus, in order to transfer a finer pattern, a KrF laser (248 [nm]), an ArF laser (193 [nm]) and, further, an F2 laser (157 [nm]), are used as an exposure light source.
As part of a positioning method of the semiconductor manufacturing apparatus, there is a need for accurately measuring a positional relationship between a reticle, which is an original plate or a reticle stage (original plate stage) on which the reticle is set and a wafer stage (substrate stage). The most advantageous measurement method thereof is TTR measurement for simultaneously measuring the reticle and the stage. The TTR measurement is measurement carried out via a projection lens located between the reticle and the stage. For an illumination light source used in the TTR measurement, exposure light is the most suitable. The reason is that aberration of the projection lens (such as chromatic aberration) is adjusted to the exposure light, which allows the reticle and stage to be simultaneously measured.
Presently, a main illumination apparatus, which can emit light with high energy and a short wavelength, is an apparatus with an excimer laser, or the like, as a light source. Such a laser is a pulse light emitting laser (pulse light emitting apparatus).
An image capturing apparatus of a pulsed laser is disclosed in Japanese Patent Laid-Open Nos. 3-226187 and 5-190421, and the apparatuses disclosed in the specifications use the following four methods to generate images with reduced illumination non-uniformity.
(1) The illumination non-uniformity of the laser is restrained by oscillating means in an illumination apparatus.
(2) The laser is synchronized with a picture synchronizing signal input in the image capturing apparatus and is controlled to have the same number of pulses during light storage.
(3) In order to reduce the illumination non-uniformity, captured electrical signals are integrated.
(4) A cycle of the oscillating means is synchronized with the cycle of image capture.
FIG. 11 is a schematic view of a configuration of a light receiving apparatus according to a conventional example. Light of a pulse laser (Laser) 14, which is a pulse light emitting apparatus, is leveled (made uniform) by oscillating means 7, such as a wedge, and after passing through mirrors 4, 5 and a half mirror 6, illuminates a mark of a wafer 3 on a substrate stage via a projection lens 2. After passing through the mirror 5 and half mirror 6 via the projection lens 2, the reflected light from the mark image is imaged by a CCD camera (cam) 8, which is a storage-type position sensor. A synchronizing signal of the CCD camera 8 is generated by synchronizing signal generator (Sync) 15. At the same time, the synchronizing signal is sent to the oscillating means 7 and laser (Laser) 14 to synchronize the CCD camera 8, oscillating means 7 and laser 14.
In FIG. 11, reference numeral 1 denotes a reticle; 9, driving means (motor); 10, an interferometer (inter); 11, a stage control apparatus (SF); and 12, an exposure control apparatus (com). Further, reference numeral 13 denotes an oscillating control apparatus (IS Cont); 16, an A/D converter; and 18, a control section for an image processing apparatus.
The CCD camera 8, which is of an NTSC system, stores light divided between even/odd timing, and, as shown in FIG. 12, an oscillating cycle is adjusted to a cycle corresponding to an integral multiple of even/odd fields. FIG. 12 is an explanatory view of timing of the oscillating means, laser light emitting and image storing, according to the conventional example.
In the conventional example, stored image data are added by an adder (sum) 17 shown in FIG. 11, and in FIG. 12, images of three or six frames are combined to generate images for measurement.
However, scan exposure has come to be carried out, which has caused the need for synchronizing the oscillating means with a scanning speed. That is, in the scan exposure, a resist on the wafer is irradiated with the light of the pulse laser as if a slit scanned over the wafer (substrate). In order to carry out exposure without illumination non-uniformity within the scanning area, exposure must be carried out in such a manner that a certain point on the wafer is irradiated with pulse light for one cycle or n cycles (n: natural number) of the oscillating means in a time period during which the point moves across the width of the slit. Thus, an increased scanning speed requires increased oscillation frequency of the oscillating means. The scanning speed is inversely proportional to energy for exposing the resist on the wafer, and an increased amount of exposure requires an increased number of laser pulses (energy). The oscillation frequency of the laser is fixed (generally largest), so that a reduced scanning speed controls the oscillation frequency of the laser. In this way, for accommodating the scan exposure, the oscillating means must change an oscillation amount (oscillation frequency) in accordance with the scanning speed (exposure amount).
In the case of storing the pulse light by the CCD camera of the NTSC system, the exposure time is limited to 1/60 second. When the storage time is limited, oscillation by the oscillating means must be adjusted to an integral multiple of 1/60 second in order not to produce illumination non-uniformity and not to cause an even/odd difference at any time in imaging by an interlace system with even/odd time divisional specific to the NTSC system.
There is an optimum oscillation frequency requested in accordance with terms of the scanning speed, while the oscillation frequency must be adjusted separately in accordance with terms of the measurement, and each measurement requires control of the oscillating means. Generally, for changing in a short time an operation speed of an object moving at a high speed, a control time of about a few seconds is required under the influence of inertia. In order to reduce the time to a few milliseconds, control means with high performance must be used. For this purpose, there is also a configuration which has oscillating means dedicated to measurement separately from the oscillating means for scanning.
However, the problem of the configuration is that the size of the illumination apparatus is increased and that double optical members for forming each oscillating means are required. Further, part of the light emitted from the light source must be directed to an optical system dedicated to measurement, which reduces illumination intensity for pattern exposure. Accordingly, the optimum configuration is such that part of an illumination system of a scan exposure system is utilized without making a dedicated optical system.
The TTR measurement is a measuring system, which is used in calibration of a stage position and a reticle position, calibration of a projection lens, or the like, and the measurement is carried out using wafer replacement time, or the like. However, a recent exposing apparatus has the shortest wafer replacement time to increase throughput (wafer processing capacity per unit of time). In the measurement carried out in the wafer replacement, dead time of the apparatus is used until the oscillating means is stabilized.
The conventional system has a problem that the oscillating means must be controlled for image capture for measurement, which has an influence on the throughput of the apparatus. When using, in the image capturing apparatus, a camera of the type that light storage divided between even/odd fields, such as the NTSC system is carried out, a difference in brightness (difference in illumination intensity) occurs between even/odd fields switched per 16.6 [msec] (= 1/60 sec). The following items are the causes of concern of occurrence of the difference in the illumination intensity.
(1) The difference in the illumination intensity occurs under the influence of a variation of laser energy in 16.6 [msec]. Especially, an amount of laser energy for a first pulse is relatively high and the amount is transitionally stabilized.
(2) The difference in the illumination intensity occurs by non-uniformity of the oscillation frequency of the oscillating means.
Harmful influence of the occurrence of the difference in the illumination intensity is poor accuracy of measurement of the captured image. For example, in measurement for quantifying a defocus amount by contrast of captured signals, accurate measurement cannot be achieved without a constant amount of light. This is because the contrast value is varied by brightness.
Thus, reduction of the difference in the illumination intensity is required for improvement of the measurement accuracy. For this purpose, there are conventional methods, including a method for increasing time (number of times) for integrating captured electrical signals and a method of discarding an image first captured by a camera. However, these methods have problems of requiring a lot of time to capture images.
Another method is such that starting points of a capture start and the oscillating means are synchronized for each even/odd field. This method has a disadvantage of an increased time to capture images and also of complicated control of the oscillating means and capture.
Another problem of time-series capture of the even/odd fields is that all picture elements are not stored at the same time. A reticle stage and a wafer stage are synchronously controlled, but when a first position of 1/60 [sec] is different from a latter position of 1/60 [sec], leveled light storage is not carried out, but the images are changed in a stepping manner.
As a summary of the above descriptions, the prior art has the problems as described below.
(1) The oscillating means cannot be adjusted to the cycle of the image capture time in a short time. Adjustment over a long time has influence on the throughput.
(2) Capturing the image by the NTSC system causes non-uniformity of amounts of illumination light between even/odd fields, which has influence on the measurement accuracy.
(3) Capturing the image by the NTSC system has no synchronism between even/odd time-division images, so that the occurrence of a fine positional change prevents generation of integrated signals.